HOW TO: Locking Down Ubuntu
Security is an important issue in computing. Unfortunately, many computers allow a cracker to gain 
access to them and retrieve sensitive information, or just make life hard. This article will review the basics in general security and explain how to apply it to two Linux distributions–Ubuntu and Kubuntu.
This article assumes that you know how to install programs on either Ubuntu or Kubuntu. It also assumes that you have some knowledge of basic computer networking principles. If you do not know how to install programs on Ubuntu, go tohttps://help.ubuntu.com/community/InstallingSoftware. If you do not know much about networking, go to http://www.faqs.org/docs/linux_network/x-087-2-intro.html. This article also assumes that you are using Ubuntu or Kubuntu 6.06(Dapper Drake), but the Firewall section can be adapted for any recent Linux distribution. Locking Down Ubuntu - Linux Forums